J. Phys. Chem. B 2002, 106, 3329-3332
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COMMENTS Comment on “Identification of Precursor Species in the Formation of MFI Zeolite in the TPAOH-TEOS-H2O System” Christopher T. G. Knight*,† and Stephen D. Kinrade‡ School of Chemical Sciences, UniVersity of Illinois at Urbana-Champaign, 600 South Mathews AVenue, Urbana, Illinois 61801, and Department of Chemistry, Lakehead UniVersity, Thunder Bay, ON P7B5E1, Canada ReceiVed: June 18, 2001; In Final Form: NoVember 15, 2001 Introduction Recently, three papers1-3 by a group of researchers from Leuven University appeared in this journal, addressing the synthesis of MFI- and MEL-type zeolites. Underpinning the work is the tacit acceptance of a widely held view of the mechanism governing zeolite nucleation and growth in solution. Specifically, the authors claim that the synthesis medium contains novel polycyclic silicate anions, variously described as “precursor species” or zeolite “molecular fragments”, which readily self-assemble to yield the final crystal structures. The idea that zeolites are formed by the sequential addition of small, prefabricated building blocks termed “secondary building blocks” (SBUs) was first suggested by Barrer in 1959.4 However, the notion that such structures actually exist in solution as discrete, stable molecules has been viewed with skepticism, even by Barrer himself.5 Indeed, when the actual structures present in aqueous alkali metal and alkylammonium silicate solutions were determined by NMR spectroscopy in the 1980s (Figure 1), zeolite SBUs were conspicuous by their almost complete absence.6-10 As one of us has observed, “The open, flexible, five-, six-, and eight-membered ring systems of the SBUs are unknown in aqueous solution. On the contrary, silicate anions tend to be as highly condensed as possible, with many containing the three membered ring, a structure unheard of in tectosilicate frameworks.”11 The Leuven group claims the existence of a number of extraordinary oligomeric silicate species, the only apparent reason being that these entities are logical “molecular fragments” of the resulting zeolite architecture. The experimental evidence that they provide, however, is insufficient to support any such structural assignments. By their own admission, the authors have accepted the premise that prefabricated zeolite structural units exist in solution and use their data to justify this assumption. This a priori acceptance of the veracity of the theory they are trying to investigate is a clear example of circular reasoning and highlights just how entrenched this concept remains in the field of zeolite science. Spectroscopic Analysis of Aqueous Silicate Solutions Infrared Spectroscopy. After mixing a droplet of tetraethoxysilane (TEOS) with one of tetrapropylammonium hydroxide * To whom correspondence should be addressed. E-mail: Knight@ silicatesolutions.com. † University of Illinois at Urbana-Champaign. ‡ Lakehead University.
Figure 1. Aqueous silicate structures identified by 29Si NMR analysis.6-10 Each line in the stick figures represents a tSi-O-Sit siloxane linkage.
(TPAOH) solution, the authors observe a decrease in intensity of an IR signal at 650 cm-1 in favor of a broad composite feature at 600 cm-1 (see Figure 1 of ref 2). They contend that these absorptions correspond to three-ring and five-ring silicate units, respectively. The former assignment is based on the NMR observations discussed below. The justification given for the latter is that solid pentasil zeolites reportedly yield an IR absorption at 550 cm-1.12 The authors argue that the 50 cm-1 difference “can be ascribed to the small size of the silicate molecule containing the five rings, and the lack of connectivity of the five rings with a framework”, thus illustrating that they have already assumed the presence of five-ring structures in solution and are simply using their data in an attempt to justify this assumption. In fact, the bands around 600 cm-1 could easily arise from four-rings or indeed from any number of structural units existing in solution. Literature investigations of tetraalkylammonium silicate solutions have focused on absorptions above ca. 1000 cm-1 and unanimously attribute such vibrations to the cubic octameric cage (“double four-ring”, species 19 in Figure 1).13 Silicon-29 NMR Spectroscopy. Silicon-29 NMR spectra of aqueous silicate solutions are notoriously difficult to assign. Silicate solutions often contain many small, rapidly interchanging oligomeric silicate anions, the equilibrium distribution of a given system being dependent on concentration, pH, temperature, and the presence of other solute species.10 At natural isotopic abundance, 29Si NMR spectra consist of a series of apparently unconnected, closely spaced singlets. Rapid exchange of the hydroxy protons removes all evidence of 1H-29Si scalar coupling. Speculation based upon relative chemical shifts and intensities of groups of signals can sometimes provide clues as to structure, but the resulting interpretations are necessarily tentative, and the lack of appropriate model compounds for
10.1021/jp012286f CCC: $22.00 © 2002 American Chemical Society Published on Web 02/28/2002
3330 J. Phys. Chem. B, Vol. 106, No. 12, 2002 chemical shift comparisons is a serious drawback. The only way to obtain direct structural information is to prepare the solutions with materials that are heavily isotopically enriched in 29Si. The effects of 29Si-29Si scalar coupling then provide connectivity data, which yield the molecular structure of the anions present through the application of various spin perturbation NMR techniques such as homonuclear decoupling or two-dimensional NMR. Through the use of these methods, the structures of all of the principal silicate anions found in aqueous alkaline solution have now been determined (Figure 1),6,9 as has the cation’s influence on silicate speciation.14-16 Four observations are relevant here. First, silicate anions will be as condensed as possible, as may be seen in Figure 1, where fully half of the anions contain the three-membered siloxy ring. Second, all structures present in tetraalkylammonium (TAA) silicate solutions are also found in alkali metal silicate solutions, although the concentrations and 29Si chemical shifts of individual species may be significantly affected. The most dramatic example of cation control over silicate speciation is the dominance of the cubic octamer polyanion (species 19 in Figure 1) in solutions prepared with tetraalkylammonium cations having alkyl group chain lengths of three or less. Third, addition of organic solVents to TAAsilicate solutions driVes the equilibria toward the cubic octamer, eVen when the TAA alkyl chain length exceeds three. Because of the hydrolysis of the tetraethoxysilane (TEOS) starting material used by the authors, their reaction systems are ethanolic tetrapropylammonium (TPA) and tetrabutylammonium (TBA) silicate solutions, both of which are known to contain high proportions of the cubic octamer.15,16 (See Figure 3 of ref 16a.) Fourth, the cubic octamer forms at an anomalously slow rate in TAA-silicate solutions.15,16 As its concentration increases, that of the prismatic hexamer (species 13) (as well as several other cage-like anions) also changes, passing through a maximum in the early stages of this process, a phenomenon that is clearly evident in Figure 2 of ref 2 (cf. Figure 1 of ref 16b). The Leuven group base their conclusions on the NMR analysis of just two sample compositions “prepared by adding 9 g TEOS to 7.9 g of concentrated TPAOH solution, either at room temperature (r.t.) or 0 °C.” They make no use of isotopically enriched materials and rely exclusively on 29Si NMR chemical shifts and relative peak intensities to derive their assignments. They use a relatively low frequency (300 MHz) spectrometer, and the spectra shown are generally of poor quality, both in terms of peak dispersion and signal-to-noise ratio (2 or less for some key peaks). Despite a heavy reliance on published chemical shifts to identify species, the spectra are referenced only to external TMS. External NMR referencing is problematic with aqueous silicates owing to large spectral shifts (up to 2 ppm) caused by variations in solution ionic strength, pH, or temperature.10a,14,16a The silicate monomer (species 1) resonates at about -71 ppm with respect to TMS and is commonly used as an internal reference. The authors do not even mention this species and edit their spectra to exclude its signal. The 29Si NMR signal of the glass NMR tube and probe insert can also be used as a very rough secondary reference, but this too has been excised from the spectra without explanation. Consequently, it is impossible to make any definitive shift correlations with literature data, or indeed even between the authors’ own spectra, because identical data sets are displayed with different ppm axes (cf. Figures 2b and 3 of ref 2). Additionally, sample conditions listed in the figure captions are often inconsistent with those given in the figure labels (see Figures 4, 5, and 6 in ref 2). Finally, the authors
Comments claim that signals from certain silicate oligomers exhibit a unique phase anomaly that arises because “samples contained variable amounts of residual octane from the TEOS extraction procedure”. Just how such a mysterious species-specific phase anomaly arises is not discussed. Silicate Speciation Known Structures. The authors propose a total of 10 silicate anions in aqueous alkaline TPAOH silicate solutions.2 Five of the anionssthe linear trimer (species 3), cyclic trimer (6), bicyclic pentamer (10), prismatic hexamer (13), and tricyclic hexamer (15)sare well-attested constituents of alkaline silicate solutions. The chemical shifts given are, however, not always self-consistent. The literature shows unequivocally that the peaks around -80 ppm arise from the dimer (2) and other species (4, 6, 9) with Q1 end groups, but here, they are all assigned to a single Q2 group in the bicyclic pentamer,17,18 the dimer not even being mentioned. Also, the separation between the Q2(c) and Q3(b) peaks claimed for the bicyclic pentamer is only about one-third that quoted in the literature. A small peak at around -88.8 ppm is assigned to the prismatic hexamer, ignoring the larger signal at -89.3 ppm, which is instead attributed to overlapping Q2 and Q3 peaks from an octameric pentacyclic species (vide infra). In fact, the signal at -89.3 ppm is clearly attributable to the prismatic hexamer on the grounds of its transient intensity (refer above to observation four) and its relative chemical shift. Thus, of the 23 previously identified silicate species shown in Figure 1, the authors consider only five. Species they do not consider, but which most certainly occur in their system, include the monomer (1), dimer (2), cyclic tetramer (8), substituted cyclic trimer (6), singly and doubly bridged cyclic tetramer (11, 12; of which the characteristic signals around -86 and -93 ppm are readily apparent in Figures 2 and 3 of ref 2), the tricyclic hexamer (14), pentacyclic heptamer (17; characteristic peak at -91.4 ppm), hexacyclic octamer (18; characteristic peak at -92.6 ppm)19 and, most significantly of all, the cubic octamer (19). These species easily account for the peaks that the authors use to identify their novel silicate structures, discussed below. Unknown Structures. Existence of the five remaining structures claimed by the authors2sthe pentacyclic octamer, tetracyclic undecamer, “33-mer”, double five-ring, and capped double five-ringshas never been established in solution. Indeed, two have never been proposed before. All five violate empirical observations concerning silicate anion structure, especially in solutions containing tetraalkylammonium cations,14-16 in that they possess large, open-framework structures that would easily rearrange into species that are smaller, more rigid, and, in the case of the undecamer and “33-mer”, more symmetric. No direct experimental evidence is offered to support their existence, and the signals are assigned solely on chemical shift and intensity grounds, although, as we show below, even these minimum criteria are sometimes not met. Again, none of the other 18 known silicate species are even considered. Instead, peak assignments corresponding to the anticipated precursor species are forced from the spectra. (i) The pentacyclic octamer’s three silicon sites, a Q2 and two inequivalent Q3 groups, would be expected to give rise to three signals with an intensity ratio of 1:1:2. The authors propose that the Q2a and Q3b signals are exactly coincident and, therefore, that the two signals at -89.3 ppm and -90.1 ppm (with an intensity ratio of 3:1) confirm its presence. They justify the coincidence of the Q2a and Q3b signals by analogy to data published for the pentacyclic heptamer (18).6b,6c In 1988,
Comments however, Knight9 showed that the heptamer’s Q2 and Q3 signals are actually separated by about 1 ppm. Moreover, as noted above, the signal at -89.3 ppm is very probably that of the prismatic hexamer. Thus, there is very little basis on which to claim the existence of the pentacyclic octamer. (ii) The tetracyclic undecamer structure would give six signals, three from its Q2 groups and three from its Q3 groups, with an intensity ratio of 1:2:2:2:2:2. The authors assign signals at around -89 ppm and -97 ppm to the Q2 and Q3 groups, respectively, while ignoring all published assignments for these two regions. In light of all that is now known of aqueous silicate chemistry, it is odd that such a large, unwieldy, and unsymmetrical structure, bearing no resemblance to any known aqueous species, could be proposed without concrete experimental evidence. (iii) The “33-mer” is claimed to be formed from three tetracyclic undecamers. Although the authors assert that the IR spectrum in Figure 1 of ref 2 “suggests the presence of a molecular fragment of this [MFI-type] framework structure”, the data shown cannot possibly support structural assignments to discrete entities in solution. The NMR evidence is similarly unconvincing, in essence being that (a) signals near -101 ppm in Figure 7 of ref 2 might be due to Q4 groups and (b) the ratio of integrated peak areas Q2/Q3/Q4 in this spectrum roughly correlates with the cumulative ratio of corresponding Si sites in the “33-mer”, thereby implying that no other species exist in solution. No attempt is made to assign the NMR signals to individual Si centers within the 33-mer. In actuality, their spectrum is similar to that of any TAA-silicate solution prepared under similar conditions (cf. Figure 1 of ref 16a) and is readily interpretable in terms of several small, highly condensed silicate anions. Studies conducted using 29Si-enriched materials and germanosilicate solutions show unequivocally that the peaks labeled as -89.3 and -100.5 ppm in Figure 7 of ref 2 correspond, respectively, to the prismatic hexamer and the cubic octamer.15,16 We also note that the chemical shift of the octameric peak (and peaks of associated species) moves to lower frequency with the addition of either TAA or organic cosolvent and, in freshly prepared solutions, as the octamer concentration steadily rises.16 (iv) The double five-ring is assigned to a peak at -98.8 ppm in Figure 5 of ref 2. However, the literature shows very clearly that the dominant signal in this region for (aged) tetraalkylammonium silicate solutions (as in Figures 2c, 5, 6, and 7 of ref 2) is that of the cubic octamer.6-11,15,16 As evidence for their assignment, the authors quote a review article by Engelhardt and Michel,20 failing, however, to note that these authors attribute peaks in this region of the spectrum to double fiverings in solids, not solutions. Indeed, no definitive evidence has ever been produced to support the existence of aqueous double five-rings.21-23 Repeated attempts at verifying its existence using isomorphic metal substitution have been unsuccessful, instead affirming that the cubic octamer and prismatic hexamer are the only double-ring cages present in solution.15,16,24 Nevertheless, along with the single five-ring, single six-ring, and double sixring species, the double five-ring continues to be claimed as a component of aqueous silicate solutions simply because it is a logical zeolite precursor species.25 (v) As for the capped double five-ring, as with the double five-ring, NMR studies of isotopically enriched silicate solutions have never revealed any evidence of this species, even in tetraalkylammonium silicate solutions. If it were to exist, the 29Si NMR spectrum would be expected to show four signals, with an intensity ratio of 1:2:4:4. The authors claim that three
J. Phys. Chem. B, Vol. 106, No. 12, 2002 3331 signals at -88.7, -99.0, and -98.7 ppm, with an intensity ratio of 1:2:8, may be assigned to it. Again, the signal at -98.7 ppm is evidently that of the cubic octamer, and the signal to low frequency of this is probably that of the singly protonated TPAclathrate of this species.24 In Figure 5 of ref 2, the authors assign the most intense peak in the spectrum at -98.8 ppm to the double five-ring. In Figure 6, they assign the most intense peak, at -98.7 ppm, to Q3 units in the capped double five-ring. According to the figure labels, the spectra are of the same solution, the only difference being that the spectrum in Figure 6 was recorded at 0 °C, while that in Figure 5 was recorded at room temperature. They are thus claiming that by raising the temperature some 25 °C all capped double five-rings are converted to double five-rings. No evidence is presented to support this remarkable conclusion. A far more plausible explanation is that the peak at -98.8 ppm in Figure 5 and that at -98.7 ppm in Figure 6 are both due to the same anion, the cubic octameric cage. As noted earlier, solution conditions can cause large variations in individual chemical shifts (complete details are provided in ref 16a), and thus, it is unsurprising that the cubic octamer resonates at slightly different frequencies in different samples. Mechanism of Zeolite Crystallization The implications of the work in terms of understanding the molecular level mechanism of zeolite formation could not be more damaging. The idea that silicate solutions contain large amounts of a single molecule such as the “33-mer”, structurally related to the zeolite that crystallizes from that solution, is completely at odds with the results of 20 years of research into solute speciation in silicate-containing systems. Silicate solutions are in fact composed of many small, highly condensed molecules in dynamic equilibrium, their relative concentrations being governed by the dictates of polymer chemistry. In the case of concentrated ethanolic tetrapropylammonium silicate solutions, the cubic octamer is known to be the dominant equilibrium species.15,16 That it is manifestly not a logical molecular fragment of MFI-type zeolites must indicate that zeolites do not crystallize from solution by the addition of preexisting building units of the requisite geometry, because no such building blocks are present in solution. The authors, however, ignore the cubic octamer altogether and instead propose a series of improbable species of which the only function is to shore up the concept of self-assembly by the condensation of ready-made “embryonic” structural fragments. In actuality, the cubic octamer is simply too small to accommodate a TPA+ cation, an observation that clearly invalidates the argument that the organic cation acts as a template in bulk solution around which a silicate anion of appropriate geometry assembles. We note elsewhere16b that in tetraalkylammonium silicate solutions it is the hydrated organic cations that encapsulate the silicate anion in solution, and not, as the authors claim, the reverse. That the Leuven group twice quote our work as support for their own highlights the very real necessity of clarifying the situation before the literature becomes further confused. Conclusion Although 29Si NMR spectra of aqueous silicate solutions at natural isotopic abundance are, in principle, consistent with the structures of an almost infinite number of silicate anions, nothing is gained by proposing novel and fanciful species that manifestly have nothing in common with known silicate anion structures. The cumulative weight of evidence indicates that there is not an unlimited number of silicate anion structures in solution, no
3332 J. Phys. Chem. B, Vol. 106, No. 12, 2002 matter what the cation, and the structures of all the major species observed in a wide variety of alkali metal and organic base silicate solutions have already been determined and reported. The distribution and nature of the silicate anions present in tetraalkylammonium silicate solutions of the sort investigated by the Leuven group have been known for many years. Yet the presence of two of the most common, and unquestioned, components of aqueous tetraalkylammonium silicate solutions, the monomer and the cubic octamer, is never once acknowledged by the authors. The unfounded structural assignments given instead only serve to demonstrate the determination with which structural fragment condensation theory is upheld,25,26 even in the face of data that clearly contradict it. We feel compelled to note that experiments aimed at determining the actual structure of silicate anions in solution have repeatedly shown that embryonic precursor species do not exist as such, no matter how appealing the concept. References and Notes (1) Ravishankar, R.; Kirschhock, C. E. A.; Knops-Gerrits, P.-P.; Feijen, E. J. P.; Grobet, P. J.; Vanoppen, P.; De Schryver, F. C.; Miehe, G.; Fuess, H.; Schoeman, B. J.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 4960. (2) Kirschhock, C. E. A.; Ravishankar, R.; Verspeurt, F.; Grobet, P. J.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 4965. (3) Kirschhock, C. E. A.; Ravishankar, R.; Van Looveren, L.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 4972. (4) Barrer, R. M.; Baynham, J. W.; Bultitude, F. W.; Meier, W. M. J. Chem. Soc. 1959, 195. (5) Barrer, R. M. Zeolites and Clay Minerals as Sorbents and Molecular SieVes; Academic Press: London, 1978; p 34. (6) (a) Harris, R. K.; Knight, C. T. G. J. Mol. Struct. 1982, 78, 273. (b) J. Chem. Soc., Faraday Trans. 2. 1983, 79, 1525. (c) J. Chem. Soc., Faraday Trans. 2. 1983, 79, 1539. (7) (a) Engelhardt, G.; Hoebbel, D. Z. Chem. 1983, 23, 33. (b) Engelhardt, G.; Rademacher, O. J. Mol. Liq. 1984, 27, 125. (8) (a) Hoebbel, D.; Garzo, G.; Engelhardt, G.; Ebert, R.; Lippmaa, E. T.; Alla, M. Z. Anorg. Allg. Chem. 1980, 465, 15. (b) Hoebbel, D.; Garzo,
Comments G.; Englehardt, G.; Vargha, A. Z. Anorg. Allg. Chem. 1982, 494, 31. (c) Hoebbel, D.; Vargha, A.; Engelhardt, G.; Usjszaszy, K. Z. Anorg. Allg. Chem. 1984, 509, 85. (9) Knight, C. T. G. J. Chem. Soc., Dalton Trans. 1988, 1457. (10) (a) Kinrade, S. D.; Swaddle, T. W. Inorg. Chem. 1988, 27, 4253. (b) Inorg. Chem. 1988, 27, 4259. (11) Knight, C. T. G. Zeolites 1990, 10, 140. (12) Jansen, J. C.; van der Gaag, F. J.; van Bekkum, H. Zeolites 1984, 4, 369. (13) Groenen, E. J. J.; Emeis, C. A.; van den Berg, J. P.; de Jong-Versloot, P. C. Zeolites 1987, 7, 474 and references therein. (14) Kinrade, S. D.; Pole, D. L. Inorg. Chem. 1992, 31, 4558. (15) Knight, C. T. G.; Kirkpatrick, R. J.; Oldfield, E. J. Chem. Soc., Chem. Commun. 1986, 66; J. Am. Chem. Soc. 1986, 108, 30; 1987, 109, 1632. (16) (a) Kinrade, S. D.; Knight, C. T. G.; Pole, D. L.; Syvitski, R. T. Inorg. Chem. 1998, 37, 4272. (b) Inorg. Chem. 1998, 37, 4278. (17) The Qy symbol is commonly used to denote a quadrifunctional Si center with y coordinated SiO44- groups. (18) All 29Si chemical shifts cited from ref 2 are taken from Figures 8 and 9 therein. Note, however, that the values are sometimes inconsistent with data shown in other figures. (19) The observation of 29Si NMR peaks that are characteristic of the hexacyclic octamer would suggest that the authors’ solutions are contaminated by alkali-metal cations.14 (20) Engelhardt, G.; Michel, D. High-Resolution Solid-State NMR of Silicates and Zeolites; Wiley: New York, 1987. (21) Engelhardt, G.; Hoebbel, D.; Tarmak. M.; Samoson, A.; Lippmaa, E. Z. Anorg. Allg. Chem. 1982, 484, 22. (22) Boxhoorn, G.; Sudmeijer, O.; van Kasteren, P. H. G. J. Chem. Soc., Chem. Commun. 1983, 1426. (23) Groenen, E. J. J.; Kortbeek, A. G. T. G.; Mackay, M.; Sudmeijer, O. Zeolites 1986, 6, 403. (24) Knight, C. T.; Syvitski, R. T.; Kinrade, S. D. Stud. Surf. Sci. Catal. 1995, 97, 483. (25) Bodart, P.; Nagy, J. B.; Gabelica, Z.; Derouane, E. G. J. Chim. Phys. 1986, 83, 777. McCormick, A. V.; Bell, A. T. Catal. ReV.sSci. Eng. 1989, 31, 97. Chen, S. S.; Chen, Y. W.; Chiang, A. S. T. Adsorpt. Sci. Technol., Proc. Pac. Basin Conf., 2nd 2000, 130. (26) Kirschhock, C. E. A.; Ravishankar, R.; Truyens, K.; Verspeurt, F.; Jacobs, P. A.; Martens, J. A. Stud. Surf. Sci. Catal. 2000, 129, 139. Li, Q.; Mihailova, B.; Creaser, D.; Sterte, J. Microporous Mesoporous Mater. 2001, 43, 51.
